Indirect vs Tankless Hot Water Heater?

I am installing a new propane boiler (munchkin 140 or weil-mclain ultra 150) and also need to install a new hot water heater. Which makes more sense for efficiency/cost, an indirect heater or a tankless hot water heater. Also how does one size such units?

 

A tankless can only provide hot water above a given minimum flow and below its maximum flow (BTU based so the max GPMs will vary based on seasonal incoming water temps). It also requires it's own gas line hookup and a separate exhaust vent poking out through the wall. It'll also requiring it's own maintenance. The indirect has no minimum flow and can flow as much as the piping can hold and ultimately flow at the capacity of the boiler. With an indirect there is one combustion device to maintain and it gives the boiler something to do over the summer. Can you guess which way I would suggest you go? ;-)

To get the most efficiency out of a modcon with an indirect, I'd size it (a bit) larger than a conventional electric or gas WH, use a protective mixing valve on it and also program the boiler to allow it a wider on off differential so that it does fewer but longer DHW cycles. The latter part of the DHW cycle will be out of range for the return waters to allow the boiler much chance of condensing. So if it's always topping up to full temp, then you won't be condensing. In addition, in the summer you will strand some BTUs in the piping between the boiler and also in the boiler's heat exchanger every time the boiler gets called, so if you can do that half as often then your wasted propane will be reduced a fair chunk - not to mention a bit less for the AC to deal with if that's the case. My opinion is that doing fewer heating cycles would help greatly in optimizing the modcon / indirect combo but I haven't seen any good research done on this.

You must have either a gigantic or poorly insulated house to require that capacity. If you haven't gone through the motions of doing a formal heatloss calculation (slantfin has a nice free heat loss calculator on their web site) then you should do that before buying the boiler.

When piping, I would actually try and give the indirect piping preference to keep the piping run as short and efficient as possible - the piping between the indirect and the boiler should be insulated since it does DHW duty in summer months where lost heat does no good at all.

My independent preference on boilers would be the Triangle Tube Prestige Solo (or Excellence if your DHW needs are smaller - it has a 14 gallon indirect and mxing valve already housed with the boiler). The Prestige can be piped direct (as opposed to primary secondary which can reduce the associated electrical costs of running your heating system) for the heating and is more of a self cleaning design. The Munchkin's Gianni (sp?) heat exchanger is quite restrictive in design and needs a fairly large pump just on the boiler loop itself. The Ultra requires closer scrutiny of the water chemistry because it has a cast aluminum block.

 

I have used the slant fin calculator and it estimate my required max heating at 123,000 BTU/hr. My house is old (130yrs) and large (3800 sqft) with limited blown in insulation and poor windows. I will do my best to resolve this - I just bought the house. The previus owner use 1600 gallons of oil per season with an 80% eff boiler. My big issue is deciding between oil and propane due to the uncertainty in fuel prices. The cost of using a 87% AFUE oil boiler compared to a 93% mod con propane - slightly favoring propane. I see the fuel selection at this point as a crap shoot. Thanks again.